1. Field of the Invention
The present invention relates to a toner supply roller for use in an image forming device, and more specifically to a developing roller including two spongy layers on its surface.
2. Description of Related Art
There has been proposed a develop unit for use in an image forming device, such as a laser printer. The develop unit includes a developing roller and a supply roller for stably and continuously supplying toner particles to the developing roller. The supply roller is usually formed from a spongy foam material. However, toner can enter the spongy foam material, so that the supply roller hardens undesirably.
Japanese Patent-Application Publication (Kokai) No. HEI-7-36273 discloses a configuration that prevents the supply roller from becoming undesirably hard. The disclosed toner supply roller has an inner layer, a middle layer, and an outer layer. Both the inner layer and the outer layer are formed from an open cell foam spongy member. However, cell density of the outer layer is smaller than that of the inner layer. The outer layer is formed with a low cell density. Therefore, there are few openings at the outer layer of the supply roller for toner particles to enter through. Further, even if toner particles enter the outer layer, few paths are available for the toner particles to reach the inner layer. Therefore, at least the inner layer remains soft, so that the toner supply roller can be maintained soft at its surface.
The middle layer is provided between the inner layer and the outer layer for further preventing toner particles from entering the inner layer through the outer layer. Any toner particles that happen to work the way through the outer layer will be blocked by the middle layer from entering the inner layer. However, providing the additional middle layer between the inner layer and the outer layer increases manufacturing costs, and also complicates processes for producing the develop unit.